The present invention relates to the cooling of high speed axial flux electrical machines.
Cooling of both the rotor and the stator in high speed axial flux machines is an important factor when considering the design of such machines. One cooling scheme adopted for cooling the rotor of such high speed axial flux electrical machines (for example as described in PCT/GB96/01292) consists of two or more discs in which the electrical coils are situated. These discs are separated by spacers consisting of thin radial wedges or spots etc. Air is pumped from the rotor rim towards the stator bore and drawn into the stator at its inner periphery via the rotor-stator air gap. The air is usually then drawn through the stator into a circumferential chamber and away by means of a fan. This kind of cooling circuit can be described as a series circuit since all the cooling air passing through the rotor-stator air gap also passes through the stator even though the cooling air for each stage of the generator is arranged in parallel.
There are a number of drawbacks with this arrangement which reduce the overall efficiency of the machine whether acting as a motor or a generator. In current designs, all the airflow must pass through the inner periphery of the stator. The gaps between the discs of the stator are preferably quite small, but this causes a severe restriction in the flow path of the cooling air. Consequently, a high pressure difference is needed in order to pass the required quantity of cooling air through the system.
This problem is made worse by the action of the rotors on the cooling air. As the air passes by the rotors between the rotor and stator, the air is accelerated in a tangential direction. This tangential air velocity sets up a vortex which causes a significant pressure differential between the outer periphery of the rotor and the stator inner periphery. In order to overcome these large pressure differentials, the fan driving the cooling air through the cooling circuit must have a high pressure ratio and consequently requires a large power input in order to drive it. This power either increases the power required to drive the electrical machine when acting as a generator or reduces the useful power available when the machine is working as a motor. In both cases the efficiency of the machine reduced.
Another problem with the series cooling circuit described above is that all the cooling air which enters the inside of the stator passes by the high speed rotor. This results in significant heating caused by the losses due to windage. This heating means the air is already hot when it enters the stator and consequently less heat is removed from the active parts of the stator. This means that because less heat can be removed from the stator, less electrical power can be drawn from or supplied to the electrical machine.
One possible solution to the problem of insufficient heat extraction from the stator is to increase the airflow through the gaps between the rotor and the stator. However this simply increases the windage losses, generating more heat in the air and requiring more power to drive the increased airflow. Consequently the machine is less efficient due to the increased power needed to provide the increased airflow.
Therefore, according to the present invention there is provided a stator winding unit for an electrical machine comprising: a plurality of disc shaped windings, at least one pair of said windings being axially spaced apart and having a spacer provided therebetween, the spacer comprising a plurality of walls between the windings defining a plurality of separate first channels for conducting cooling fluid, wherein each first channel is arranged for conducting cooling fluid between a first port at the outer periphery of the stator winding unit and a second port provided in at least one of said pair of windings.
The present invention further provides a stator winding unit for an electrical machine comprising: a plurality of disc shaped windings, at least one pair of said windings being axially spaced apart and having a spacer provided therebetween, the spacer comprising a plurality of walls between the windings defining a plurality of separate first channels for conducting cooling fluid, wherein each first channel is arranged for conducting cooling fluid between a first port at the outer periphery of the stator winding and a radial second port provided at the outer periphery of the stator winding unit.
The present invention provides a number of advantages over the series cooling circuit of the prior art. An electrical machine in accordance with the present invention allows the quantity of air passing through the gap between the rotor and the stator to be substantially less than the amount of air passing through the inside of the stator. This provides a number of advantages over the series cooling circuit of the prior art. As the amount of air required to pass between the stator and the rotor is reduced, the pressure difference required to pump that air is also reduced and consequently less power is needed to pump the air. This helps to improve the overall efficiency of the machine. Not all the cooling air passing into the stator has been preheated by passing between the stator and rotor and so the overall temperature of the air entering the stator is lower and thus the air is able to remove more heat from the stator. This means that more power can be supplied to or drawn from the stator for a given maximum temperature limit of the stator. Since more heat is removed by a given amount of air, less air is required overall and furthermore less air is required to pass through the gap between the rotor and the stator, and so the losses will be further reduced. This leads to an overall increase in the efficiency of the machine by reducing both heating due to windage losses and the power consumption of the fan since the total cooling air mass flow required is reduced.